On one side, TWDM-PON (Time Wavelength Division Multiplexing-Passive Optical Network) has been considered as primary solution for NG-PON2 (Next Generation Passive Optical Network), where four XG-PONs are stacked and ONU transmitters/receivers are able to tune to any of the four upstream/downstream wavelengths. Within a wavelength, TWDM-PON reuses XG-PON downstream multiplexing and upstream access techniques, timeslot granularity, multicast capability, and bandwidth allocation mechanism.
On the other side, to open the access infrastructure network to the 3rd party is a hot topic and a trend. For example, TWDM-PON operator (i.e., A-InP) opens his TWDM-PON to the two 3rd parities, VNO (Virtual Network Operator) A and VNO B. That is to say, A-InP constructs two virtual PONs for the two VNOs. Different ONUs will be configured for different VNOs on demand.
To enter the normal Operation state, ONU must first perform the ONU activation process to obtain the parameters (such as ONU-ID, equalization delay, profile information). In XG-PON, once the network is ranged, and all ONUs are working with their correct equalization delays, all upstream bursts will be synchronized together between all the ONUs.
In XG-PON, there is no needed to change the receive/transmit wavelengths of ONU. In TWDM-PON, there is a need of reconfiguring the receive/transmit wavelength of ONU. Obviously, when the TWDM-PON is deployed in the actual access network, sometimes there are requirements to change some active ONUs' working wavelengths to the other ones because of the operator's some considerations. For instance,                If there is only a few active ONUs, to make the power saving, we could request the active ONUs to working on the same receiving wavelength and the same transmitting wavelength.        To make good use of broadcast of the PON, and considering the multicast/broadcast information, we can change the wavelengths of all ONUs that receiving the same multicast application to the same receiving wavelength. Thus the TWDM-PON can be more efficient.        The end-user of VNO A is attached in the A-InP's network, and he/she wants to change to other VNO (e.g., VNO B) whose virtual access network is still on the same A-InP network. The A-InP must have the capability to change the active ONU to the other correct wavelengths used for VNO B.        If there are too many active ONUs working on the same receive/transmit wavelength, to make the load balancing, then some ONUs must be adjusted to other wavelengths with light traffic load.        
Different XG-PONs in TWDM-PON system may have their own burst profile information, different ONU-ID space, default and explicit Alloc-ID allocation scheme; thus even though the same physical ONU is attached to different XG-PON in TWDM-PON, it will have different equalization delay, ONU-ID, default Alloc-ID, and different XGEM PORT-ID, burst profiles. To make an active ONU work other wavelengths, the ONU must be stopped fast from the current wavelength, and obtain the target wavelength information and also the above-mentioned parameters for the target wavelengths via activation process.
In one existing approach, a new message λ-TUNE is defined, which carries new wavelength information to ONU. This approach has the following disadvantages. First, deactivation process is performed after the λ-TUNE message, and two subsequent interchanges are needed, that is, the λ-TUNE needs whole deactivation process and whole activation process. ONU needs to record the λ-TUNE message and use information in the λ-TUNE message in the subsequent reactivation process. Second, a quiet window is needed to avoid collisions with the regular upstream bursts during serial number acquisition and ranging of newly joining ONUs. Third, a new message type ID is needed.